Process for precipitating a high melting metal contact layer at low temperatures

ABSTRACT

A method for precipitating a high melting metal contact layer, at low temperatures, through thermal dissociation of a gaseous compound of the high melting contact metal and precipitating the same upon a carrier body, preferably of semiconductor material. The metal contact layer is precipitated upon the carrier body through thermal dissociation of the easily volatile trifluorophosphine or trifluorophosphine hydride of the respective metals.

United States Patent Inventor Erhard Sirtl Midland, Mich. App]. No.886,946 Filed Dec. 22, 1969 Patented Nov. 9, 1971 Assignee SiemensAktiengesellschaft Berlin, Germany Priority Jan. 2, 1969 Germany P 19 00119.5

PROCESS FOR PRECIPITATING A HIGH MELTING METAL CONTACT LAYER AT LOWTEMPERATURES 15 Claims, 1 Drawing Fig.

US. Cl 117/227, l17/l07.2 R, 23/203 C Int. Cl C23c 11/02 Field of Search23/203 C; l 17/ 107.2 R

semiconductor References Cited OTHER REFERENCES Zeitsclir'ift furangewandte Chemie Vol. 79, No. l pages 27- 43, 1967 PrimaryExaminer-William L. Jarvis Attorneys curt M. Avery, Arthur E. Wilfond,Herbert L.

Lerner and Daniel J. Tick ABSTRACT: A method for precipitating a highmelting metal contact layer, at low temperatures, through thermaldissociation of a gaseous compound of the high melting contact metal andprecipitating the same upon a carrier body, preferably of material. Themetal contact layer is precipitated upon the carrier body throughthermal dissociation of the easily volatile trifluorophosphine ortrifluorophosphine hydride of the respective metals.

PROCESS FOR PRECIPITATING A HIGH MELTING METAL CONTACT LAYER AT LOWTEMPERATURES My invention relates to a method for precipitating a highlymelting metal contact layer, at low temperatures through thermaldissociation of a gaseous compound of the high melting contact metal andprecipitating the same upon a carrier body preferably of semiconductormaterial.

Highly purified thin metal layers with extremely high meltingtemperature or extremely low moisture can be obtained by purely vapordepositing methods, only with great difiiculty. The heretofore used gasreactions invoke a reduction of fluorides or of chlorides with hydrogenor the pyrolitical dissociation of carbonyl compounds.

The known methods all have shortcomings which greatly influence theproduction of uniform metal contact layers. Thus, reaction of a fluoridebrings about difficulties, caused by hydrofluoric acid formation duringdissociation. Reduction of the chloride requires relatively hightemperatures, for a substantial precipitation of the metal. Oxygen andcarbon are formed during the dissociation of carbonyl become installedinto the metal lattice or interfere with a homogeneous precipitation inthe form of a foreign phase.

My invention overcomes these disadvantages by precipitating the metalcontact layer through thermal dissociation of the readily volatiletrifluorophosphine or trifluorophosphine hydride of the respectivemetals upon the carrier body. The advantages obtained thereby resultfrom the fact that these compounds are:

l. easily volatile 2. substantially nonaggressive, hence to not entaildifficulties associated with the work material.

3. easily dissociate, forming thereby relatively inert phosphorustrifluoride (Pl-" and 4. easily purified, which is of importance for thequality of the precipitated metal layer.

It is within the scope of the present invention to usetrifluorophosphine compounds of the metals nickel, cobalt, iron,chromium, molybdenum, tungsten, niobium, tantalum, vanadium and/ormetals of the platinum group.

The following table gives a picture regarding the pertinent metalcomplexes which were tested more closely by Th. Kruck in Zeitschrift furangewandte Chemie, 79, 27 1967).

Sublimation or It was found to be most advantageous and to effect animprovement in the adhesiveness of the metal contact layer on thecarrier body, to subject the surface of the carrier, prior toprecipitating the metal contact layer, to a pretreatment through theaction of sulphur hexafluoride (SR) or nitrogen trifluoride (NE), atelevated temperatures, preferably between 500 and l,000 C.

According to a preferred embodiment, hydrogen and/or noble gases areused as a carrier gas for the thermal dissociation of the trifluorophosphine or trifluoro phosphine hydride of the respective metals. Thevariable decomposition of the PF; complex, due to the viscosity or theheat transfer differences of both carrier gas types, is to be taken intoaccount at otherwise equal testing conditions.

According to another embodiment it is also possible to effect thethermal dissociation of the trifluorophosphine compound at a reducedpressure, preferably in a vacuum of 1 Torr. This can be done with orwithout a carrier gas. The reaction temperature must, of course, beadjusted to the pressure conditions. One can also operate within aflowing gas system.

The temperature range of 350 to 600 C. which is required for thermaldissociation of the trifluorophosphine'compounds is adjusted throughindirect heating of a quartz table which is in thermal contact with thecarrier body. It was found preferable, to use a slit molybdenum disc asa heater, which is rinsed by argon in order to avoid an oxidating effectcaused by air.

The present invention also affords the opportunity of carrying out aselective and even an epitactic precipitation of metal layers. Thethermal dissociation is so controlled that an additional energy sourcewhich acts from the outside, eg a selective UV radiation, limits themetal precipitation to specific regions of the carrier surface. ln thismanner, all possible metal structures can be produced in a simple andrational manner, upon carrier bodies with and without masking layers.

In addition to semiconductor materials, such as germanium, silicon, orA"B" compounds; quartz or ceramic as well as metallic systems can alsobe used as materials for carrier bodies.

The temperature of the vaporizing vessel containing thetrifluorophosphine compounds, is preferably from 20 to 100 C Accordingto a preferred embodiment of the present invention, metal contact layersare precipitated at a thickness of approximately 1,000 A. These layersof metal contact are characterized through a particularly high purityand heat resistance, uniformity of the layer design and by a goodelectrical conductance.

The aforementioned qualities make the layers particularly well suitedfor the production of semiconductor device components, especially ofmetal base transistors and Schottky diodes. However, their use is notlimited to semiconductor art, as can also be used with the same goodresults for producing frontal contact layers for electrical capacitorsand resistors. Another application possibility associated with thedevice component industry, is plating radio tubes.

The single FIGURE of the drawing schematically shows a device suitablefor carrying out the invention.

The invention will be described with reference to the precipitation of atungsten contact metal layer upon a carrier body comprising a siliconsemiconductor crystal using the apparatus of the FIGURE.

A quartz tube reaction chamber 1 holds a silicon crystal substrate 2 tobe coated on a quartz table 3, into which a slit molybdenum wafer 4 isso installed as a heater that it can be rinsed during operation by acurrent of gaseous argon (flow rate 3 to 10 liter/hour), or other inertgas in order to flush out all air. The argon gas is blown in asindicated by arrow 5. The molybdenum wafter 4 is heated by the currentleads 6 and 7 and the silicon crystal wafer 2, which is processed withpure aqueous HF, is first heated to a temperature of 750 C. The

nitrogen trifluoride (NF,,) taken from a storage container 22,

situated in a branch line 21, is thinned with argon (30 l./h.), with amole ratio n(NF )/n(Ar) of 10 to 10", is passed for about 15 minutesthrough reaction chamber 1 which exposes the, pure silicon surface onthe silicon crystal wafer 2. The flow meter 23, installed in the branchline 21 and the valves 24 and 25 are used to regulate the etching gascurrent. Hydrogen is subsequently passed via flow meter 26 through a gassupply line 8 with valve 27 open, at a flow rate of 30 l./h., with acooling trap 9 (temperature bath -78 C.) and thence in a directionindicated by arrow 10, across a vaporization vessel 11. The hydrogenwhich acts as a carrier gas, thus becomes charged with thetungsten-trifluoride-phosphine (W(PF 12, contained in the vaporizationvessel 11 maintained by a temperature bath 13, at 80 C. The compound,mixed with the carrier gas is then passed via a frit or screening plate14, into the reaction chamber 1 and is dissociated at the gas-etchedsilicon carrier body 2 which is being maintained at 450 C. toprecipitate tungsten. After about 30 minutes, an approximately 1000 Athick tungsten layer of high uniformity has formed on the siliconcrystal wafer and is in tight contact with the silicon surface. With theaid of valves 15, 16 and 17, shown in the drawing, the reaction chambercan be charged, according to the position of the valves, with only purecarrier gas or with only trifluoridephosphine compounds. Valves 18 and19 assure an exact adjustment of the flow rate of the carrier gascurrent. The residual gases and the volatile reaction products, leavethe reaction chamber at the arrow 20.

The other complexes described above in the table, behave analogously.

I claim:

1. A method of precipitating a high melting metal contact layer, at lowtemperatures, through thermal dissociation of a gaseous compound of thehigh melting contact metal and precipitating the same upon a carrierbody, said contact metal layer is precipitated upon the carrier bodythrough thermal dissociation of the easily volatile trifluorophosphineor trifluorophosphine hydride of the respective metal.

2. The method of claim 1, wherein a semiconductor is the carrier body.

3. The method of claim 2, wherein trifluorophosphine compounds of ametal selected from nickel, cobalt, iron, chromium, molybdenum,tungsten, niobium, tantalum, vanadium and metals of the platinum group,is used.

4. The method of claim 3, wherein the carrier surface is subjected,prior to precipitation of the contact metal layer, to a pretreatment ofsulphur hexafluoride (SP or nitrogen trifluoride (NF at elevatedtemperatures.

5. The method of claim 3, wherein the temperature range is between 500and l,000 C.

6. The method of claim 1, wherein hydrogen or a noble gas is used as acarrier gas during the thermal dissociation of trifluorophosphinecompounds.

7. The method of claim 6, wherein the thermal dissociation is effectedat reduced pressure, preferably in a dynamic vacuum of 10 to 1 Torr.

8. The method of claim 7 wherein the pressure is reduced 10 to 1 Torr.

9. The method of claim 1, wherein the carrier body is heated to atemperature required for thennal dissociation, through indirect heatingof a quartz table in thermal contact therewith.

10. The method of claim 9, wherein a slit molybdenum wafer is used as aheater and rinsed with argon.

11. The method of claim 1, wherein the thermal dissociation is at atemperature of 350 to 600 C.

12. The method of claim 1, wherein a selective precipitation of thecontact metal layer upon the carrier surface is produced by an energysource which acts from the outside.

13. The method of claim 1, wherein the carrier body is selected fromquartz, ceramic or metal.

14. The method of claim 6, wherein the trifluorophosphine compound isvaporized at from 20 to C.

15. The method of claim 1, wherein the contact metal layer isprecipitated in thickness of 1000 A.

i i It t I!

2. The method of claim 1, wherein a semiconductor is the carrier body.3. The method of claim 2, wherein trifluorophosphine compounds of ametal selected from nickel, cobalt, iron, chromium, molybdenum,tungsten, niobium, tantalum, vanadium and metals of the platinum group,is used.
 4. The method of claim 3, wherein the carrier surface issubjected, prior to precipitation of the contact metal layer, to apretreatment of sulphur hexafluoride (SF6) or nitrogen trifluoride (NF3)at elevated temperatures.
 5. The method of claim 3, wherein thetemperature range is between 500* and 1,000* C.
 6. The method of claim1, wherein hydrogen or a noble gas is used as a carrier gas during thethermal dissociation of trifluorophosphine compounds.
 7. The method ofclaim 6, wherein the thermal dissociation is effected at reducedpressure, preferably in a dynamic vacuum of 10 3 to 1 Torr.
 8. Themethod of claim 7 wherein the pressure is reduced 10 3 to 1 Torr.
 9. Themethod of claim 1, wherein the carrier body is heated to a temperaturerequired for thermal dissociation, through indirect heating of A quartztable in thermal contact therewith.
 10. The method of claim 9, wherein aslit molybdenum wafer is used as a heater and rinsed with argon.
 11. Themethod of claim 1, wherein the thermal dissociation is at a temperatureof 350* to 600* C.
 12. The method of claim 1, wherein a selectiveprecipitation of the contact metal layer upon the carrier surface isproduced by an energy source which acts from the outside.
 13. The methodof claim 1, wherein the carrier body is selected from quartz, ceramic ormetal.
 14. The method of claim 6, wherein the trifluorophosphinecompound is vaporized at from 20* to 100* C.
 15. The method of claim 1,wherein the contact metal layer is precipitated in thickness of 1000 A.